This invention relates to printed circuit substrates incorporating resistance elements, and more particularly to a novel printed circuit substrate with resistance elements in which a resistor pattern film and a conductor (including electrodes) pattern film are incorporated in advance, whereby a printed resistor circuit board with resistance elements having high performance can be manufactured, while the number of processing steps can be greatly reduced.
A conventional printed circuit substrate incorporating a resistor, which is well known in the art, is a multilayer stock comprising an insulating support, a resistance layer combined with the entire surface of the insulating support, and a high conductive material layer combined with the resistance layer. In the case where a printed circuit board with resistors is fabricated by using this conventional printed circuit substrate, an insulating region (all the layers on the insulating support being removed), a resistance region (the high conductive material layer being removed), and a conductor region (none of the layers being removed) are formed by a subtractive method (mask-etching method).
A typical one of the prior art concerning printed circuit substrates of this type is that disclosed by Japanese Patent Laid-Open No. 73762/1973. According to this publication, a printed circuit substrate is manufactured and converted into a printed circuit board with resistors through a number of intricate processes as outlined below: One surface of a high conductive material layer such as a copper foil is covered with a removable masking sheet, and a resistance layer is formed on the other surface of the high conductive marterial layer by electrodeposition. Then, the masking sheet is removed, and thereafter an insulating support is combined with the resistance layer. Thus, the printed circuit substrate incorporating a resistor is obtained.
This substrate is further subjected to the following processes: After the surface of the copper foil of the substrate is covered with photoresist, it is subjected to exposure through a photographic negative having a conductor pattern and a resistor pattern in combination (having two patterns corresponding to the conductor region and the resistor region) and is then subjected to development, whereby the photoresist is mantained unremoved in the pattern regions. The copper foil in the region which is not covered with the photoresist is removed by etching, and furthermore the resistance layer exposed is removed by using an etching solution, as a result of which the surface of the insulating support is exposed. Then, the left photoresist is removed by using a removing solution.
Thereafter, the substrate is covered with photoresist again, and is then subjected to exposure through photohgraphic negative having the conductor pattern. The substrate is subjected to development to retain the photoresist in the conductor pattern region. The copper foil in the region which is not covered by the photoresist (or the copper foil having a configuration corresponding to the resistor pattern region) is removed by etching, as a result of which the surface of the resistance layer corresponding to the resistor pattern region is exposed. Then, the remaning photoresist is removed by using a removing solution.
A solder stop-off or the like is applied to the resistance layer in the resistor pattern region by printing, and is then heated and cured to cover the resistance layer. Thus, the printed circuit board with resistors is obtained.
However, the drawback in the above-described process is that, in the second process of cleaning and removing the remaining resist (on the conductor pattern region) by using the removing solution, the resistance layer in the resistance pattern region has been exposed already and is subject to the removing solution. The thickness of the resistance layer is made to the considerably thin, of the order of several hundreds to thousands of A, in order to increase its sheet resistance. Accordingly, its mechanical strength is very low. Especially, if it is rubbed in a resist removing solution, its combining force with the combining surface of the insulating support is reduced, which leads to damage, the insulating support being, in general, a laminated plate of epoxy resin and glass cloth. Thus, variation of the sheet resistance and lowering of a variety of characteristics are caused.
In order to overcome these various difficulties, a method has been proposed in which the conductor pattern region of the circuit substrate is protected by a gold plating film, and in the final process the copper foil having a configuration corresponding to the resistor pattern region is removed by etching. However, persons skilled in the art will readily understand that in this method the processes are rather intricate, requiring considerable skill.
As is apparent from the above description, the conventional printed circuit substrate incorporating a resistor and its processing technique suffer from various disadvantages that, because the number of processing steps is relatively many and the processes are intricate, it takes a relatively long period of time for the processing, the manufacturing cost is high, the yeild is limited to a certain extent, and the auxiliary material cost is high.
Accordingly, the printed circuit substrate of this type cannot be extensively employed in a variety of industrial fields without solving the above-described problems accompanying the conventional printed circuit substrate.